The following thesis investigates the feasibility of a microbial fuel cell (MFC) driven by thermoacidophilic methane removing biotrickling filters (BTFs) and chemical mediators. MFCs are primarily explored for wastewater treatment applications, generating electricity from an otherwise energy-consuming process. Methane is a potent greenhouse gas abundant in biogas from anaerobic digestion in wastewater treatment. Methane abatement in this industry primarily consists of flaring to less potent CO₂, due to corrosive contaminants (H₂S) commonly found in biogas. Alternatively, methane can be converted to CO2 while also generating electricity using methane-fed MFCs.

The feasibility of methane-fed MFCs to treat biogas and other waste methane sources is hindered due to poor water solubility. One strategy to improve methane removal efficiencies is to utilize BTF reactors, which employ microbial biofilms in contact with gaseous compounds to improve mass transfer. By combining BTFs with MFCs, methane abatement, and thus, power density of the technology, could be improved. However, to capture electrons released from microbial methane degradation in the BTF, a method to bridge electron transport is required. One method is to utilize chemical mediators, which bridge electron flow from BTF biofilms to the electrode.

For a mediated MFC and BTF to be feasible, a suitable methane-degrading organism with the ability to transfer electrons quickly to mediators is required. Methane-degrading organisms (methanotrophs) are ubiquitous in nature; however, thermoacidophilic methanotrophs are rarely investigated in biotechnical applications. Moreover, thermoacidophilic methanotrophs are naturally acclimatized to sulfur environments/compounds such as H₂S, which are commonly found in biogas and are inhibotry to mesophilic methanotrophs. Thus, thermoacidophilic methane removing microbial communities in BTFs were enriched, and used along with mediators for power generation in a MFC and BTF design for treating waste methane emissions.

Thermoacidophilic methane removing microbial communities were enriched in BTFs independently of MFCs, using geothermally active soil from Rotokawa, New Zealand. Four BTF reactors were operated under both microaerobic (2% O₂) and aerobic (10% O₂) conditions for 2 years, with methane elimination capacities (ECs) between 7.6–30.7 g m⁻³ hr⁻¹ under non−limiting conditions. Microbial communities in all reactors were dominated by Verrucomicrobiota thermoacidophilic methanotrophs (Methylacidiphilum). Enriched BTF communities were screened for mediator reduction, using thionine acetate, methylene blue, and neutral red, showing reduction rates up to 0.023 mmol hr-1 g-biomass⁻¹. Limiting current and spectrophotometric methods were employed to monitor mediator reduction kinetics. Thionine acetate reduction rate was superior among all mediators examined and demonstrated first-order kinetics based on electrochemical monitoring. Mediator reduction was independent of methane oxidation, and dosing with the glycolytic inhibitor iodoacetamide partially ceased thionine reduction.

Investigation of thionine in BTFs demonstrated that reactor biofilm could reduce thionine under low O₂ (< 0.1% v/v) conditions over multiple cycles. BTF microbial communities were acclimated to carbon felt anodes in a dual-chamber MFC with O₂ reduction at abiotic carbon felt cathodes. The combined thermoacidophilic BTF and MFC produced maximum power densities between 4.6−7.9 mW m⁻² and cell potentials of 0.37-0.45 V. Under micaerobic conditions BTF reactor EC was not inhibited by thionine presence; however, methane removal was undetectable during intermittent (1-5 day) O₂ starvation. The results of this thesis demonstrate a proof-of-concept of a combined BTF and mediated MFC for electricity generation and methane abatement using thermoacidophilic aerobic methanotroph communities. Moreover, electrochemical monitoring of bulk mediator reduction in suspended cultures, seeded from BTFs, revealed the dependence of mediator reduction on glycogen metabolism. These findings guide further development of methane-fed MFCs utilizing BTFs and reveal the microbial mechanisms driving mediator reduction in suspended communities.